Video observation of an eel in the Anguilla japonica spawning area along the West Mariana Ridge

Some anguillid spawning areas are known based on collections of small larvae, but recently for the Japanese eel Anguilla japonica, adult spawners have been caught in trawls and their eggs and preleptocephali collected. The spawning area of A. japonica is located along the western side of the West Mariana Ridge, but the natural spawning behavior of this species or that of any other anguillid species has never been observed. This study reports on the first effort to observe spawning aggregations of anguillid eels that was conducted by the R/V Yokosuka using the Shinkai 6500 submersible and a Deep-Tow camera system in the A. japonica spawning area in July 2012. The submersible was deployed mostly at 200–800 m during daytime and the Deep-Tow was deployed mostly at 130–250 m during nighttime, both in multiple oblique depth tracks along linear transects. Various fishes and invertebrates were seen in the pelagic environment during day and night, but no spawning aggregations were observed. One eel was briefly recorded by a Deep-Tow camera at 20:13 on 17 July (2 days before new moon) at a depth of 179 m. The eel was recorded for <1 s as it passed in front of the camera. Its anterior body and head shape were consistent with a male A. japonica, or possibly a Derichthys serpentinus eel, but not with other mesopelagic eels. Because the tail region of the eel was not visible, species identification was not possible.     

Discovery of a spawning area of the common Japanese conger <Emphasis Type="Italic">Conger myriaster</Emphasis> along the Kyushu-Palau Ridge in the western North Pacific

The common Japanese conger Conger myriaster is an important commercial coastal fisheries species in East Asia, but its spawning area has not been determined. A larval sampling survey was conducted in September 2008 along 136°E between 13°N and 22°N, which roughly followed the Kyushu-Palau Ridge in the western North Pacific. Twenty larval specimens were confirmed to be C. myriaster using DNA analysis. Two were newly hatched larvae (preleptocephali) 5.8 and 7.8 mm in total length (TL), which were caught at 17°N. The 5.8 mm TL larva was estimated to be 3–4 days after hatching, the youngest preleptocephalus (i.e., the earliest stage) of this species ever collected. Eighteen other leptocephali were caught at 18°N and 21°N, and these ranged from 18.6 to 40.0 mm TL. Based on these collections, we discerned that there is a spawning area of C. myriaster in the area along the Kyushu-Palau Ridge approximately 380 km south of Okinotorishima Island. Similar to the Japanese eel spawning area along the West Mariana Ridge, the Kyushu-Palau Ridge may play an important role as a landmark of the spawning area. The discovery of this offshore spawning area should lead us to a better understanding of the recruitment mechanisms of C. myriaster, and help to facilitate future international management efforts.     

Several possible spawning sites of the Japanese eel determined from collections of their eggs and preleptocephali

The spawning area of the Japanese eel is located at the southern part of the West Mariana Ridge in the western North Pacific, but their spawning events have not been observed. To further understand Japanese eel spawning ecology, an interdisciplinary research survey by the R/V NATSUSHIMA (NT14-09, 14 May–4 June 2014) was conducted to detect spawning sites based on the seamount, salinity front, new moon and third quadrant (spawning south of front, west of ridge) hypotheses. Attempts were made to film spawning events with underwater camera systems and to consider if eels might be detected in hydroacoustic observations. Although no Japanese eels or spawning events were video-recorded and no eel aggregations could be clearly identified acoustically, three eggs were collected at two stations in the third quadrant region at or just south of 13° N on 26 and 27 May. Three or four days later, newly hatched preleptocephali were collected at two stations far to the south, including 224 at a station > 160 km southwest of the egg catches, and a few preleptocephali were caught at two stations closer to the egg stations. The eggs and southern preleptocephali were from discrete spawning events, which indicated that at least two spawning sites occurred in May 2014.

     

Genetic identification and morphology of naturally spawned eggs of the Japanese eel Anguilla japonica collected in the western North Pacific

Eggs of the Japanese eel Anguilla japonica collected in the western North Pacific were identified by onboard species-specific polymerase chain reaction (PCR) and DNA nucleotide sequencing after the cruise. Fish eggs of various species were collected by large plankton net tows at 12 stations along the southern part of the West Mariana Ridge on 19–25 May 2009. A total of 43 fish eggs were distinguished morphologically as possibly being of A. japonica. Thirty-one of those were analyzed by PCR, which included 15 eggs collected at 12°50–55′N, 141°15–20′E (in 5 tows) that showed positive results. The 16S ribosomal RNA (rRNA) gene sequences of eggs determined after the cruise indicated that 31 A. japonica eggs had been collected. The remaining eggs were of mesopelagic eel species (Serrivomeridae and Derichthyidae), or unidentified species. The morphological characteristics of the A. japonica eggs were consistent with those of artificially spawned eggs, except they had a slightly larger diameter. The egg diameter range did not overlap with those of mesopelagic eels of the Serrivomeridae, which often spawn in the same area as A. japonica. These results suggest that egg diameter and embryo shape can be used to morphologically identify naturally spawned A. japonica eggs.     

Evaluation from otolith Sr stable isotope ratios of possible juvenile growth areas of Japanese eels collected from the West Mariana Ridge spawning area

Fisheries Science - The widely distributed East Asian Japanese eel Anguilla japonica constitutes a single genetically homogeneous population with a single spawning area near the West Mariana Ridge....     

日本鳗鲡产卵后生存及其繁育

为证明日本鳗鲡(Anguilla japonica)生活史最后一步一产后鳗的命运,本研究模拟产后的日本鳗鲡继续在海水中养殖,观察其存活率及繁育情况.结果表明,产后鳗在海水中停食约18 d后,体能得到恢复,部分亲鱼开始出现摄食,1个月左右全部恢复摄食,经244 d养殖,雌、雄鳗体质量增加,存活率达94.6%.随后,给产后鳗注射外源性促性腺激素(鲤鱼脑垂体匀浆CPE和人绒毛膜促性腺激素HCG)后激发其退化的性腺(卵巢和精巢)重新发育(与当年银鳗作对照).通过性腺组织切片观察产后鳗和对照鳗性腺发育成熟的全过程及其差异,发现产后鳗起初性腺发育比当年银鳗差,但经多次注射激素后,产后鳗性腺成熟与当年银鳗同步,证明产后鳗生殖细胞对激素的敏感件高.应用17α,20β-双羟孕酮和促性腺激素释放激素类似物(GnRH-A3)使催熟的产后鳗和对照鳗均产卵和排精,并孵化出仔鱼,从而有力地证明,鳗鲡产后虽体质弱,但待体能恢复后能够继续生存和繁殖.本研究旨在探讨利用产后鳗作为今后鳗鲡人工繁殖亲鳗的可行性.     

Demographic survey of the yellow-phase Japanese eel Anguilla japonica in Japan

Catch statistics and biological data of the Japanese eel Anguilla japonica from 12 river systems in nine prefectures of Japan (36°N, 140°E–31°N, 130°E) during 1999–2004 were analyzed in order to evaluate the demography of A. japonica in Japan. Significant differences in biological characteristics of 6388 eels were found among the river systems. Fishery catches of eels in all locations have declined, but the magnitudes or patterns of decline seem to be different. Of sex-identified eels, mean total lengths ± SD (mm) of females (n = 3776) and males (n = 962) were 495.6 ± 104.3 and 412.9 ± 80.7, respectively, and overall sex ratio (% female) was 79.6 %. Mean age (years) and growth rate (mm year^?1) were 5.0 ± 1.9 and 96.7 ± 38.6 for females (n = 3643) and 3.6 ± 1.7 and 120.5 ± 65.4 for males (n = 907), respectively. This study highlights the demographic heterogeneity of the A. japonica among the studied river systems, which suggests that it is important to conserve a variety of habitats in multiple river systems as an inclusive management target in addition to restoration of the diversity of habitats for eels in a single river system.     

Decreasing dietary lipids improves larval survival and growth of Japanese eel Anguilla japonica

Shark eggs-based diet is the only diet by which eel larvae can grow to glass eels in captivity. However, the high level of lipids in the diet is suggested to negatively affect eel larvae. This paper examines the effect of defatted shark eggs (DSE) and hen egg yolk (HY) on growth and survival of larvae of Japanese eel Anguilla japonica. Lyophilized shark egg and commercial HY were defatted with n-hexane, and four experimental diets were prepared using both defatted and untreated shark eggs and HY. Larvae were reared for 3 weeks by feeding the experimental diets. The highest survival rate was observed in the larvae fed DSE, and larvae fed HY showed the lowest survival rate. The best growth was found in larvae fed DSE, followed by shark eggs and defatted HY, and the worst growth was in HY-fed larvae. These results show that decreasing dietary lipids improves the nutritional value of both shark eggs and HY for eel larvae. Regulation of the dietary lipid level may positively affect the larval performance of eels by combination of ingredients with a low lipid content.     

Synthesis and possible function of 11-ketotestosterone during oogenesis in eel (Anguilla spp.)

The control of 11-ketotestosterone (11-KT) biosynthesis and its physiological roles were examined in female Japanese eel (Anguilla japonica) and New Zealand longfinned eel (Anguilla dieffenbachii). 11-KT was detected in serum of female eels of both species. Among various tissues from Japanese eel, the ovary had the greatest capacity to synthesize 11-KT in vitro. In addition, the oocyte diameters of eels treated with 11-KT had increased significantly. Furthermore, these oocytes were found to have an increased number of oil droplets. These findings suggest that 11-KT in female eels may be mostly of ovarian origin and that this androgen appears to play an important role in controlling pre-vitellogenic oocyte growth.     

Predation on glass eels of Japanese eel <Emphasis Type="Italic">Anguilla japonica</Emphasis> in the Tone River Estuary,Japan

Aquaculture of Japanese eel Anguilla japonica relies upon the natural recruitment of their glass eels (juveniles); however, predation that could influence glass eel recruitment remains unknown. In the present study, we aimed to elucidate the proportion of predation on A. japonica glass eels through stomach content analysis of predatory fishes collected in the estuary region of the Tone River system and its vicinity in Japan. Species of the predated glass eels were identified by DNA barcoding. A total of 270 predatory fishes of 15 taxa was collected over 2 years. The overall proportion of predation on glass eels, genetically identified as Japanese eel, was 0.7%, but this rose to 2.0% when data were limited to fishes caught during the peak months of glass eel recruitment. A glass eel was found in the stomach contents of a channel catfish Ictalurus punctatus, an invasive species in this river system, and a blackfin sea bass Lateolabrax latus. These fishes are therefore considered potential predators of A. japonica glass eels. However, as the proportion of predation was low, and the glass eels represented only small proportions of predator stomach contents, further investigation is needed for a better understanding of predation on A. japonica glass eels, and its effects on the early life stages of this endangered species.     

The first success of glass eel production in the world: basic biology on fish reproduction advances new applied technology in aquaculture

The eel has long been esteemed as an important food fish in the world, especially in Japan, and has been used as an experimental fish for many fields of fish physiology. However, the decreases in eel resources have been a serious concern in recent years. The catches of glass eels as seedlings for aquaculture have shown a long-term decrease in both Europe and East Asia. To increase eel resources, the development of techniques for artificial induction of maturation and spawning and rearing their larvae have been eagerly desired. Recent progress of reproductive physiology of fish, especially mechanisms of oocyte maturation and ovulation in female and of spermatozoa maturation in male, facilitate to establish techniques for hormonal induction of maturation and spawning in sexually immature eels. With persistent effort to development of rearing techniques of larvae, we have first succeeded to produce glass eel. These applied techniques are may contribute to understand the basic reproductive physiology of the eel.     

Gonadal maturation and spawning migration of stocked female barfin flounder Verasper moseri off the Pacific coast of northern Japan

Commercial catches of barfin flounder Verasper moseri, an important target flatfish for stock enhancement programs in northern Japan, have recovered remarkably since the large-scale release of seedlings from Hokkaido in 2006. However, their reproductive ecology remains unclear. Our study of 2008–2012 investigated seasonal changes in the maturity of stocked female barfin flounder (n = 4,123) off the Pacific coast from Hokkaido to Tohoku (35.7°–43.4°N). Histological observation of ovaries revealed that fish undergoing vitellogenesis were observed mainly off Hokkaido and partially off northern Tohoku during August–January (40.2°–43.4°N). However, fully matured or spawning fish were not found near those areas. They were observed only in southern Tohoku. Ongoing spawning was observed off southernmost Tohoku (35.8°–37.5°N around 300 m depth) from early February through late April, when commercial landings had decreased drastically off Hokkaido. Spent fish were caught only rarely off southern Tohoku, but were observed frequently off Hokkaido during April–June. These results demonstrate that spawning grounds form at the upper continental slope off southernmost Tohoku during February–April. Relations between maturation traits and seasonal landings strongly suggest that female barfin flounder repeatedly migrate more than 700 km from feeding grounds off Hokkaido to spawning grounds off southern Tohoku.     

Evidence of niche segregation between freshwater eels and conger eels in Kojima Bay, Japan

The aim of this study was to examine the possible interspecific interactions between the freshwater eel Anguilla japonica and the marine conger eel Conger myriaster. Spatial distribution, foraging time, and diet were compared between 380 A. japonica and 221 C. myriaster juveniles in a large brackish water bay and a tributary river estuary. Almost no C. myriaster were caught in the estuary, but both species coexisted in the bay at similar densities. For both species, percentages of eels with stomach contents were greater in the morning than in the afternoon, indicating they forage at night. Both species mainly fed on benthic crustaceans in the bay, but the size of the main prey species, mud shrimp Upogebia major, was significantly larger in A. japonica. The difference in prey size appears to be related to the eel body size (A. japonica: 559.5 ± 88.0 mm, C. myriaster: 356.4 ± 56.0 mm), which probably reflects the differences in the life history-related movements of these eels, with A. japonica using the bay at large sizes and C. myriaster using the bay as a nursery area before moving to deeper water. The differences in their migratory and movement patterns may result in niche segregation in the brackish bay where both species coexist.     

Effects of environmental variables on recruitment of <Emphasis Type="Italic">Anguilla japonica</Emphasis> glass eels in the Yangtze Estuary,China

The recruitment pattern of Anguilla japonica glass eels into the Yangtze Estuary, China was studied during five successive migration seasons (2012–2016). The variation in daily catch was analyzed in relation to environmental factors that affect inshore migration. Two large migration waves of glass eels were observed in each recruitment season: one in the winter (January/February) and the other in the spring (March/April). A generalized additive model indicated that daily changes in glass eel catch were partially affected by local water temperature and tidal range. Water temperature seemed to have a threshold effect in winter, and there were optimal temperature ranges of 6–8 °C and approximately 10.5–12.0 °C associated with different recruitment waves to the Yangtze Estuary. The influence of tides was complex, with an overall positive correlation. The annual glass eel catch fluctuated greatly, but generally did not show a significant downward trend. To achieve sustainable use of glass eels, it is proposed that relevant management and conservation activities be reinforced.     

Evaluation of the pigmentation stages and body proportions from the glass eel to yellow eel in Anguilla japonica

The pigmentation stages of Anguilla japonica were classified from the glass eel to yellow eel stage using 412 wild-caught eels from Hamana Lake and detailed developmental observations of 10 laboratory-reared eels. The sequential appearance of pigment was similar to that in A. anguilla, except for delayed pigment formation on the nerve cord of A. japonica. The general classification of pigmentation stages (V_A–VI_B) in A. anguilla was applicable to A. japonica, but the V_B stage could be separated into two stages (V_B1, V_B2) in A. japonica. The completion of guanine deposition on the intra-abdominal membrane may be a trait to discriminate the yellow eel stage (VII) from the VI_B stage. In natural conditions, wild-caught 0-age eels (n = 3,298) did not increase in size and retained a slender body form until VI_A4, and then became thicker during VI_B. The transition between body forms and growth may correspond to their ecological change from using passive transport in glass eels with tidal flow into rivers, to their settlement and further movements of elvers upstream. Considering this behavioral transition and pigmentation progression, it is proposed that the terms "glass eel" and "elver" could be defined as stages V_A–VI_A4 and VI_B, respectively.     

Freshwater eels: A symbol of the effects of global change

Temperate eels Anguilla anguilla (European eel), A. rostrata (American eel) and A. japonica (Japanese eel) are three catadromous species which have been declining since the 1970s/1980s despite their remarkable adaptive capacity. Because of their specific life cycles, which share distant oceanic spawning grounds and continental growth stage, eels are affected by five components of the global change: (a) climate change affecting larval survival and drift, (b) an increase in pollution leading to high levels of contamination exacerbated by their high lipid levels, (c) increasing fragmentation and habitat loss that reduce dramatically the amount of available habitats and induce increased spawner mortality, (d) the appearance of Anguillicola crassus a parasitic alien nematode that impairs spawning success, and (e) the impact of commercial and recreational fisheries for all life stages of eel. In this context, the rapid increases of pressures during the “Great Acceleration” have surpassed the adaptive capacity of eels. This illustrates that cumulative effects of global change can lead to the collapse of species, even in species that have amazingly high adaptive capacities.     

Fluctuation in the distribution of low-salinity water in the North Equatorial Current and its effect on the larval transport of the Japanese eel

Surface water in the North Equatorial Current (NEC) is composed of southern low‐salinity water diluted by precipitation to less than 34.2 psu and northern, high‐salinity tropical water greater than 34.8 psu. Analyses of 27‐year historical data, observed in winter and summer along the longitude 137°E by the Japan Meteorological Agency, shows that an obvious salinity front (34.5 psu) generated by the two water masses was usually located around 15°N. However, the salinity front has been moving northward during the past three decades. El Niño/Southern Oscillation (ENSO) affected salinity in the surface layer, while temperature changed in the middle layer. The salinity front sometimes moved southward, mainly south of 5°N, and the movement was well correlated with the southern oscillation index (SOI). Because precipitation at Yap (9.5°N, 138.1°E) fluctuated with SOI, this spike‐like southward movement of the salinity front was probably affected by reduction of low‐salinity water during El Niño in the north‐western Pacific Ocean. However, ENSO only induced such large southward movements of the salinity front when the time lag between the low precipitation and low SOI was short (within four months). This salinity front is quite important for long‐distance migrating fish such as the Japanese eel because the eels spawn just south of the salinity front in the NEC. This behaviour suggests that the movement of the salinity front associated with ENSO may control the success of larval transport from the spawning ground in the NEC to the nursery ground in East Asia. In fact, catch of the Japanese eel larvae in Japan was well correlated with fluctuation of SOI and the location of the salinity front, and lower catch occurred during El Niño. The salinity front has moved from 13°N to 17°N during the past three decades. Considering that conditions of larval transport are worse north of 15°N, we suggest that decadal‐scale linear decrease of glass eel catch during the past three decades also can be explained by the displacement of the salinity front.     

The first production of glass eel in captivity: fish reproductive physiology facilitates great progress in aquaculture

Weekly injections of salmon pituitary extracts (SPE) were administered to female Japanese eel, Anguilla japonica at a dose of 20 mg/fish. This induced vitellogenesis and caused oocytes to reach the migratory nucleus stage. Later, a majority of the females that received an injection of SPE at a priming dose, followed 24 h later by 17,20β-dihydroxy-4-pregnen-3-one (DHP), ovulated 15 to 18 h after the final injection. In cultivated males, repeated injections of human chorionic gonadotropin (hCG) at a dose of 1 IU/g BW/week induced spermatogenesis and spermiation. Since potassium ions were revealed to be an essential constituent for the maintenance of motility in the eel spermatozoa, artificial seminal plasma containing KCl was designed as a diluent of milt, and enabled the preservation of milt for several weeks in refrigeration. As a result, artificial fertilization performed immediately after ovulation with pre-diluted and stocked milt consistently resulted in the production of high-quality gametes. Recently, a slurry-type diet made from shark egg yolk has been found to be a suitable feed for captive-bred eel larvae. Although preleptocephalus larvae can be reared with this diet beyond the depletion of their yolk and oil droplet stores, it remains inadequate because larvae reared under this way cannot be raised to the following stage. Therefore, the diet was improved by supplements of krill hydrolysate, soybean peptide, vitamins and minerals. Larvae fed on this new diet have grown to 50 to 60 mm in total length (TL), and have begun to metamorphose into glass eels approximately 250 days after hatching.     

Controlled reproduction in the wild European eel (Anguilla anguilla): two populations compared

This study aimed to describe the response variability of female silver eels in terms of gonad development and eggs production to a standardized gonadotropic treatment (Carp pituitary extracts—CPE), and to relate this variability to population characteristics. For this purpose, sexual maturation, ovulation, and fertilization were induced in two eel populations coming from different locations in Adriatic Sea (Comacchio—CM and Marano-Grado—MG lagoons), and after that, their reproductive capacity was valuated. External (Silver index—SI, Eye index—EI, Pectoral fin length index—PFLI, Condition factor—K) and hormonal (17β-estradiol—E2, testosterone—T) parameters were measured, and some subject/group were killed for histological and lipid analysis and age determination. Morphometric parameters showed the CM-Group to have highest values of Body weight (BW), Body length (BL), and K, while MG-Group presented highest PFLI and Gonadosomatic index (GSI) values. Regarding hormonal analysis, the CM-Group showed significantly higher T and E2 levels than the MG-Group, both groups showed considerably rapid increase at T5 (5th injection). A positive trend in gonadal development was found through histological evaluation; a more regular maturation was observed in the MG-Group, whereas the CM-Group presented an exponential oocytes development starting from T10 (10th week), which led to an anticipated spawning. Lipid content showed significant differences in T0 (start study), post-ovulation, and Control (30th week) between CM and MG eels. As to zootechnical performances, while MG eels released spontaneously into the water, the CMs were stripped in order to check ovulation. The MG eels were statistically the most productive with 40.1 ± 6.33 % BW of eggs released. Furthermore, CM females ovulated mainly between the 19th and 22nd week (77.8 % spawned eels) instead in the MG’s ovulation goes from the 24th to the 28th week (100 % spawned eels). As fertilization is of concern, in both groups fertilized eggs were obtained with no difference in larvae production. These results seem to indicate that bigger dimensions, higher K, and larger lipid content (Comacchio eels) could fasten gonadic maturation without positively influencing reproductive performance of animals, both in term of quantity and quality of produced eggs. Smaller females with a highest SI (Marano-Grado eels) presented a more regular gonadic development, leading the animals to spontaneous spawning.     

Late arrival of Anguilla japonica glass eels at the Sagami River estuary in two recent consecutive year classes: ecology and socio-economic impacts

To determine the actual status of the recruiting glass eel stock of Anguilla japonica, we conducted a continuous monthly monitoring program for 2?years between November 2009 and October 2011 at the Sagami River estuary, Japan. A total of 114 and 372 A. japonica glass eels were observed in the 2009?C2010 and 2010?C2011 seasons, respectively. Recruitment patterns were the same in both years, starting in early winter (December in 2009 and November in 2010), increasing slightly until March, decreasing in April, but attaining the largest peak in June, after which no glass eels were observed during August?COctober. The Japanese eel has been known to spawn mainly in the summer, and glass eels recruit to their freshwater growth habitats during the winter to early spring. Our results clearly demonstrate an unexpected late arrival of glass eels in the early summer for two recent consecutive year classes. The summer recruitment found in our study indicates the unusual phenology of the Japanese eel, which may be a possible response to recent climate change.